The CHD-04-YIV head part can be unlocked during the mission. To unlock the part, follow the following directions:

This part can be found immediately below the main bridge near the mission starting point. To reach this area, jump down to one of the side bridges and walk to where it ends. Turn to the opening below the main bridge, and land on the nearby scaffold. There should be a large gap existing between this scaffold and the area below the starting point. Begin flying in that direction and land on the highway. You will find the part at the edge of this highway. After obtaining the part, complete the mission to unlock it.

Afterwards, fly back onto the top of the bridge and begin flying forward. eventually you will find another section of the bridge with the remainder of the enemies. Be careful when landing, because the left side of the bridge will be slanted, and will fall away when you land on it.